Direct esterification of carboxylic acids with phenols substituted with electron withdrawing groups is not typically possible due to the deactivation effect that the electron withdrawing group has on the phenolic oxygen. Methods to overcome this deactivation effect include activating the carboxylic acid by converting it into a carboxylic acid chloride or a carboxylic anhydride. Carboxylic acids can be converted to carboxylic anhydrides by reacting the carboxylic acid with a lower carboxylic anhydride, such as acetic anhydride, at elevated temperatures. However, this method suffers from high cost and volume of waste associated with producing and using acid chlorides, or the higher cost of the additional steps required to prepare carboxylic anhydrides other than acetic anhydride. Additionally, the carboxylic acid to be activated cannot contain reactive functional groups such as alcohols, thiols, amines, and amides since such functional groups are known to react with anhydrides to form undesirable side products.
European Patent EP 0 355 384 A1 discloses a process for preparing acyloxybenzenesulfonic acid or its salt by reacting hydroxybenzenesulfonic acid and a carboxylic acid in the presence of acetic anhydride. Acetic acid is removed continuously from the reaction. The carboxylic acid is a hydrocarbon which does not contain any reactive functional groups other than the primary carboxylic acid group which reacts with acetic anhydride.
Another method to overcome the deactivation effect of an electron withdrawing group on a phenolic oxygen is to first acylate a phenol with a low molecular weight anhydride, such as acetic anhydride, and then perform a transesterification reaction with the acylated phenol and a carboxylic acid. The disadvantage associated with this method is the additional steps required to produce the acylated phenol. U.S. patent application Ser. No. 08/294,236 discloses a five step method for preparing an purified alkali metal salt of 4-sulfophenyl-[(1-oxyalkanoyl)amino]alkanoate wherein an acylated phenol intermediate is formed.
Accordingly what is needed is a process to prepare amido ester compounds, which have perborate-activating properties, without the necessity of preforming an acylated phenol intermediate. In addition, the process to prepare amido ester compounds should avoid the formation of undesirable side products.